At the present time, hemodialysis or plasmapheresis blood chambers, so called air-trap blood chambers or "drip chambers" are typically made from two injection molded parts, comprising a round top cap which is solvent bonded to cylindrical chamber. Flow typically proceeds from a blood tube access port in the top cap out through an exit port at the bottom of the chamber. This structure has the following problems:
1. The two part assembly of top cap to cylindrical chamber is expensive and prone to leak, and the residual solvent adhesive is potentially toxic to the blood. PA1 2. These top caps and chambers are generally round in cross section for injection molding and assembly reasons. Otherwise leaks will occur even more frequently. However, the round shape leaves barely sufficient room for the three to four ports which must communicate through the top cap to the interior of the drip chamber. These access ports must fit typically within a 15-18 mm. O.D. (outer diameter) area of the top cap, eliminating the chance of directly connecting a pump segment to a drip chamber, which typically is a 12 mm. O.D. tube placed in a port having an O.D. of about 14 mm. Arterial drip chambers are always in close proximity to a pump segment but, because of the above, the drip chamber must be connected to the pump segment via a pump segment connector attached to a length of blood tube, the end of which can fit on the top cap. This is an expensive solution, also prone to leaks and high residuals of solvents. PA1 3. Drip chambers are placed in arterial bloodlines either downstream of the pump segment ("post-pump") or upstream ("pre-pump"), depending on the prescription of the physician and the type of dialysis machine. Pre-pump arterial chamber bloodlines are more expensive to make because the IV saline port must often be mounted on a separate "T" connector upstream from the drip chamber. On "post-pump" bloodlines the IV saline port can be mounted on the inlet pump segment connector, thus saving one part and one tube and the assembly thereof. This combination connection also reduces leaks and solvent residuals. PA1 a. During the priming procedure prior to dialysis, the arterial tubing upstream from the IV saline port must be retrograde primed. A drip chamber in this segment is difficult to retrograde prime. PA1 b. During dialysis, saline infusion (for relief of hypotension) is most safely done if any entrained bubbles are caught by a downstream arterial drip chamber. Also, the saline flow can only be visualized if there is a drip chamber downstream. PA1 c. During rinse-back of blood to the patient at the end of dialysis, the arterial fistula and the arterial blood tube upstream from the IV saline port must be retrograde flushed with saline to return this blood to the patient. To counteract the resistance of blood pressure, the saline bag is typically squeezed to create retrograde saline flow. This resistance is much greater if a drip chamber is upstream from the IV saline port. (Note: rinse-back of the downstream portion of the arterial and venous lines is done by the blood pump so resistance in this direction is unimportant). Further, retrograde rinse of blood tubing is desirably of "plug flow" type, resulting in little saline being added to the patient. If a drip chamber is upstream from the IV saline port, the blood in the drip chamber is diluted slowly by saline, resulting in large amounts of saline being administered to the patient. This is a problem, since one of the goals of dialysis is to remove fluid from the patient. PA1 The inlet port enters the blood space at a point higher than the outlet port, and PA1 There is a diversion means to prevent inlet flow from breaking the surface of the blood space and causing foaming, such diversion directing the flow in the direction of the blood outlet. PA1 a. Unlike Heath, only one end of the pump segment need be tethered to the cassette, again reducing cost of construction. The pump segment is formed straight, and is curved by the curve of the stator of the pump housing. Heath, on the other hand, is curved into a backwards C by the presence on the cassette of two transverse mounted pump segment connectors. This complicates the molding and assembly methods required. PA1 b. Unlike upside-down U pumps, the described U pump segment primes easily. Pump segments able to provide high bloodflow rates have large inner diameters, in the range of 8 mm or more. In order not to crush delicate blood cells, peristaltic pump rollers are calibrated to leave a small gap between the pump segment walls when being crushed by the rollers. This gap, however, leaks air quite readily making it difficult for enough vacuum to be created to lift the initial column of saline up to prime the pump segment. With a U pump segment, gravity causes the saline to fall into the pump segment, thereby priming it. PA1 c. A U blood pump easily allows bottom-entry, bottom exit arterial chamber, which is well known to handle rapid bloodflows with less turbulence and foaming than top-entry chambers.
There are a number of reasons why the IV saline port location is different in "pre-pump" and "post-pump" bloodlines, but each relates to the necessity of administering saline upstream from the arterial drip chamber:
One partial solution to the problems of arterial chamber has been the use of blow-molding to make one piece chambers. Thus, the two part assembly problems discussed above are eliminated. The other problems remain.
Also, the blowmolded chambers in the literature are all so-called "bottom entry" chambers whereby the blood inlet port is at the bottom of the chamber and blood enters into the blood space at the bottom or sidewall of the chamber. (This is opposite to "top entry" chambers, all injection molded so far, where blood enters at or adjacent the top into the airspace above the blood.) Two problems of the bottom entry chambers as disclosed in Swan U.S. Pat. No. 4,681,606, Heath U.S. Pat. No. 4,668,598 and European Patent Application No. 0058325A1 are:
The first problem is that blood must often be "rinsed-back" to the patient (at the end of dialysis) in a retrograde direction from the dialysis flow. With the inlet higher than the outlet, some blood will be caught in the chamber that cannot be returned to the chamber (the amount determined by the volume contained between the inlet and outlet).
The second problem is that any entrained air in the inlet blood stream is directed toward the outlet, which under certain circumstances or today's higher blood flows can escape. As the primary function of the chamber is as an air trap, this is a significant problem.